Why It Matters

If it’s unchecked, climate change will cause widespread environmental, economic, and social damage.

Carbon conundrum: One way to decrease the amount of carbon in the atmosphere would be to reverse the kind of deforestation shown here in Victoria, Australia.

A U.N. climate report released on Sunday concludes that there may still be time to limit global warming to an increase of two degrees Celsius or less, which could help the world avoid the worst effects of climate change. But doing so will depend on making extraordinary changes to energy infrastructure at a much faster pace than is happening now, and may require the use of controversial and unproven technologies for pulling greenhouse gases out of the atmosphere.

The new report, to be released in full tomorrow, is the third in a series issued in the last year by the U.N.’s Intergovernmental Panel on Climate Change. The first of these three reports looked at evidence for climate change, while the second investigated the effects of climate change and options for adapting to it. The new report considers options for preventing climate change by reducing greenhouse gas emissions. Together, the three IPCC reports constitute the group’s fifth climate assessment, synthesizing tens of thousands of scientific studies to guide government policy makers on climate change.

The new report is mostly concerned with the extent and pace of greenhouse gas emissions and what it will take to reduce those emissions to limit climate change. But its conclusions are partly based on the premise that it will be possible to extract greenhouse gases from the atmosphere—a technology that has yet to be proven on any practical scale.

“There’s a whole suite of technologies that are fun to talk about, but nothing’s ready for prime time,” says David Victor, professor of international relations and director of the Laboratory on International Law and Regulation at the University of California at San Diego.

Scientists generally agree that avoiding the worst of climate change will require stabilizing levels of key greenhouse gases below 450 parts per million (greenhouse gas levels, including carbon dioxide and other gases such as methane, are already at 430 parts per million). But even with aggressive measures to reduce emissions—investing in energy efficiency, renewable energy, nuclear power plants, and technology to capture carbon dioxide from fossil fuel power plants—the world is likely to shoot past that amount. As a result, limiting warming to two degrees Celsius might require actually removing greenhouse gases from the atmosphere, to bring levels back down to 450 parts per million.

At this point, it’s clear that climate change can’t be prevented entirely. It has already had an impact on sea levels, ocean acidification, and many ecosystems (see “Why We Can’t Just Adapt to Climate Change”). And the new IPCC report concludes that emissions of greenhouse gases have increased faster in the last 10 years than in the three previous decades. The report says the worst effects of climate change may be avoided with substantial investment in technologies to reduce greenhouse gas emissions. But the report also mentions three ways to take carbon dioxide out of the air, and all are problematic. The first is increasing the number of trees on the planet to absorb more carbon dioxide, but this requires reversing a long trend of deforestation.

The second is an approach called bioenergy with carbon capture and storage, or BECCS. This involves growing trees and other biomass, burning it to generate electricity, and then capturing the carbon dioxide this releases and storing it underground. In theory, this could reduce carbon dioxide levels in the atmosphere. But the scale of this approach could be limited in practice; it may be hard to generate large amounts of electricity using trees without causing major deforestation. And carbon capture and sequestration technology has not been proven at a large scale (see “What Carbon Capture Can’t Do” and “Capturing and Storing Carbon Dioxide in One Easy Step”). The report acknowledges that this approach is risky. “My own view is that BECCS is a fantasy,” says Victor. “It’s something the modeling community came up with because it allows the models to solve seemingly unsolvable problems.”

Finally, the report also refers to other “carbon dioxide removal” technologies, such as experimental approaches that use various materials to absorb carbon dioxide. Because carbon dioxide occurs in very low concentrations in the atmosphere (only a few hundred parts per million) capturing it usually takes a lot of energy. As a result it will likely be costly. “It’s a lot cheaper to prevent emissions than to try to remove them later,” says Howard Herzog, a senior research engineer at the MIT Energy Initiative.

Even if all greenhouse gas emissions were stopped today, the gases that have already accumulated likely will cause more warming for decades until the world’s climate settles into a new equilibrium. The fact that carbon dioxide accumulates in the atmosphere, and elevated levels can persist for hundreds of years, means that actions taken now will have long-lasting effects.

The report says that cost estimates for reducing greenhouse gases vary widely, in part because it’s not clear which technologies will work, how much they will cost, and how fast they can be deployed. Using one baseline set of assumptions, the report estimates that addressing emissions will reduce global consumption by between 2.9 and 11.4 percent by 2100. But if carbon capture and storage technology cannot be deployed, for example, costs could more than double. It’s also possible that reducing climate change may in itself have significant economic benefits, but these benefits are not included in the report’s cost estimates.